1. Technical Field of the Invention
The present invention relates to a GPS positioning system (Satellite positioning system) in which a positional coordinate can be found by receiving radio wave from a satellite, and a data server for GPS positioning.
2. Prior Art
Kinematic positioning in a GPS is a survey method where a pair of antenna and a receiver are arranged in a fixed manner at a reference point whose positional coordinate is known and another pair of movable antenna and receiver perform measurement in a short time while sequentially moving among a large number of survey points.
There exists a real-time kinematic positioning (hereinafter, referred to as an RTK) as a developed type of the kinematic positioning. The RTK is a positioning method where a position being a measurement result can be obtained in a real-time.
As shown in FIG. 6, a fixed station 2 that consists of antenna and receiver is arranged at the reference point whose coordinate is known and a mobile station 1 that consists of an antenna and a receiver performs measurement while moving it sequentially.
The RTK simultaneously receives radio wave from a plurality of satellites 8 with the fixed station 2 and the mobile station 1, analyzes positioning data in the mobile station 1 referring to positioning data obtained in the fixed station 2, and thus a relative coordinate from the known point of the fixed station 2 to a measurement point of the mobile station 1 immediately can be found.
Further, for transmission of the positioning data from the fixed station 2 to the mobile station 1, method of transmitting data from the fixed station 2 by particular frequency radio is used. Specifically the fixed station 2 was provided with a radio transmitter (one having the frequency of 400 MHz and the output of about 10 mW, for example) to transmit constantly the positioning data, and the mobile station 1 was equipped with a radio receiver capable of receiving the transmitted radio wave, and thus it has been able to refer to the transmitted positioning data as needed.
On the other hand, survey result of the GPS requires various kinds of correction according to a geodetic system and environment.
Specific correction items are listed as follows.
(1) Geoid correction (Correction of the geodetic system)
Geoid is ‘One that matches mean sea level out of the equipotential surface of the gravity of the Earth’. Height used in public survey in Japan, that is, the Japan geodetic system is an elevation having the mean sea level (geoid surface) of Tokyo bay as a reference.
On the other hand, one that the GPS uses as a reference is the Earth ellipsoid (WGS-84), and the height to be found (WGS-84 system) is the height from the ellipsoid surface as well.
The Japan geodetic system and the WGS-84 system have the difference due to different definitions between them as large as about 50 m in height depending on an area. Therefore, a coordinate system needs to be converted in order to use a positional coordinate obtained by the GPS survey as the survey result in the Japan geodetic system.
Since the geoid has fine unevenness, an approximate ellipsoid of revolution is generally fitted to the Earth's surface, which set as a reference ellipsoid, and the distance of a perpendicular to the ellipsoid is generally set as an ellipsoid height (h). Note that the Bessel ellipsoid is used as the reference ellipsoid in Japan.
Incidentally, assuming h is the ellipsoid height and H is the elevation, it follows that ‘h=H+N’ (refer to FIG. 4), and N in the equation is called a geoid height.
Accordingly, the elevation H can be obtained by subtracting the geoid height N from the ellipsoid height h obtained by the GPS positioning.
The geoid height and parameter for coordinate system conversion are found for each area, and by obtaining the geoid height and the coordinate conversion parameter that correspond to the area, positional coordinate value that conforms to the Japan geodetic system can be obtained from the positioning result by the GPS.
(2) Information regarding satellite
Although satellites used for the GPS have been launched to cover all over the Earth, their existence density or the like is scattered. In the case where satellites gather in a portion of an all-sky viewed from an antenna, in other words, when the positions of the satellites whose radio wave is receivable in the all-sky are distributed unevenly, measurement accuracy in analyzing a measurement position deteriorates.
Furthermore, when an obstacle or the like exists around the antenna, a satellite may exists from which radio wave cannot be received depending on the time.
Conventionally, before setting out for a measurement operation, arrangement information of the GPS satellites has been processed on a personal computer or the like after the obstacle viewed from an observation point was predicted, and thus the time unsuitable for the measurement operation had to be set.
(3) Information regarding ionosphere and weather
Furthermore, since the RTK performs analysis based on radio wave from the satellite, which is received by both of the fixed station and the mobile station, the influence to the accuracy of a coordinate value found by the analysis becomes measurable if the condition of the ionosphere and the atmosphere, in which each radio wave passes until it reaches the Earth, changes considerably.
For this reason, taking in consideration the condition of the earth viewed from the satellite, a range having the radius of about 10 km around the fixed station is generally set as a range in which the condition of the ionosphere and the atmosphere, in which radio wave from the satellite passes, is regarded to be substantially the same. This range is a range where the mobile station can refer to the predetermined fixed station (hereinafter, a fixed station reference range) to maintain measurement accuracy as the RTK.
Consequently, a correction value for influence caused by the condition of the ionosphere and the atmosphere in the route, where radio wave passes until it reaches the earth, is transmitted to each mobile station, and thus the RTK positioning can be performed even outside the fixed station reference range.
Correction items concerning the condition of the ionosphere and the atmosphere are called ‘The atmosphere model’.
Specifically, it corrects the traveling direction and the difference in speed of radio wave caused by the condition of the atmosphere that exists between the satellites and the observation points (both of the fixed station and the mobile station), which is the density or the like, for example. By applying ‘the appropriate atmosphere model’ between the satellites and the mobile station and the fixed station severally, propagation speed of radio wave can be accurately known and the accuracy of positional analysis for the mobile station can be thus improved.
Correction regarding the weather information is also important to improve positioning accuracy as a similar one.
Basically, each correction operation in the GPS has not been performed during the positioning operation, but it has generally been prepared before the operation based on a plan or correction has been made to data after the positioning operation.
However, unlike static positioning performed by fixing the antenna, in kinematic positioning having the premise that the positioning operation is performed while moving the antenna, there have been many cases where positioning operation was difficult at a point where positioning was originally planed or where an operator could not deal flexibly with changes of the environment and the condition at an operation site because they are difficult to predict.
Further, when a problem is found in the positioning result as a result of performing correction of the geoid or the like, its verification is impossible in the case where a cause of error positioning is based on the site condition. Accordingly, the problem due to the same cause is very likely to happen even if the positioning operation is performed again, and there is no way to recognize it.
In addition, when radio is used in transmitting the positioning data, the receiving frequency of the receiver was required to previously synchronize with a transmitting frequency of the fixed station to be referred to before the positioning operation.
On the other hand, in the case where the positioning operation is performed on the RTK, a fixed station had to be newly installed or the positioning operation had to be performed with reduced accuracy when a planed point of positioning is off from the fixed station reference range.
Moreover, there exists a ‘virtual reference station’ method in which highly accurate RTK can be performed even outside a reference-possible range of each fixed station. However, the ‘virtual reference station’ that is a virtual fixed station must be set in advance in a predetermined range set by a plurality of the fixed stations, and treatment of the ‘virtual reference station’ is substantially the same as that of the foregoing fixed station.